This application is based upon and claims the benefit of priority of Japanese Patent Applications No. H.11-95437 filed on Apr. 1, 1999 and No. 2000-44097 filed on Feb. 2, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention:
The present invention relates to a cold-warm working and heat treatment method of high carbon-high alloy group steel, in particular, a Hot Isostatic Pressing treatment at high temperature and at high pressure to be executed after cold-warm forging process of high carbon-high alloy group steel so that cracks and voids of first stage carbide formed by the cold-warm forging may be effectively cured, thus, effectively preventing a reduction of machinery strength and a deterioration of sliding wear resistance due to the cracks and voids of the first stage carbide.
2. Description of Related Art:
Parts and components made of high carbon-high alloy group steel and applicable to, for example, machinery parts, automobile parts and so on, have been manufactured in a manner that column or square pillar shaped base elements are processed, after hot forming or machining working, through annealing, hardening and tempering treatment. If the parts and components made of high carbon-high alloy group steel are processed through cold working such as cold forging which is commonly applied to mass-produced parts and components, plenty of hard and large grain first stage carbides are crystallized so that not only the working die may wear out but also the parts and components thus processed are likely to be cracked due to the cracks and voids of the first stage carbide that are inevitably formed. Therefore, it has been considered difficult to apply the cold working such as cold forging to the parts and components mentioned above.
However, recently, owing to the development of die wear resistant steel or lubricant or the supply of parts and components having good dimensional accuracy and soft material characteristic, some of the parts and components are manufactured by the cold-warm forging at less than 600xc2x0 C. temperature. But, as it is difficult to completely prevent the formation of cracks and voids of the first stage carbide, the cold-warm forging process mentioned above is implemented only on the parts and components applicable to a limited area where the deterioration of machinery characteristic is acknowledged to be allowable and the existance of the cracks and voids of the carbide is not so risky.
Further, keeping step with necessity of high pressure fuel supply system for vehicles and high speed working machinery, there is a recent tendency that highly rigid parts and components are strongly demanded and, if possible, such parts and components are fabricated by cold-warm forging in view of cost reduction.
The present invention has been made to solve potential problems in view of the above mentioned demand, and an object of the present invention is to provide a cold-warm working and heat treatment method of high carbon-high alloy group steel having a characteristic that, when a component made of high carbon stainless steel or high carbon-high alloy group steel is fabricated by cold-warm working such as forging with an aim of higher productivity and lower cost, cracks and voids of first stage carbide formed by the cold-warm forging may be effectively cured. As a result, with respect to the component so processed, a drawback such as a stick of full hard carbide to the sliding portions thereof may be solved, while the air tightness may be assured and the machinery characteristic may be improved.
To achieve the above object, the method comprises the steps of executing cold-warm working at a temperature of less than 800xc2x0 C. to make more than 10% plastic deformation with respect to a component having plenty of crystallized first stage carbides and made of at least one kind of steel of high carbon-high alloy steel group including high carbon stainless steel and tool steel so that cracks and voids may be formed in the crystallized first stage carbide and executing a Hot Isostatic Pressing treatment on the component so that the cracks and voids in the first stage carbide may be cured.
The high carbon stainless steel contains more than 0.5 mass % carbon and more than 8 mass % chromium, and the tool steel contains more than 0.5 mass % carbon and more than 0.5 mass % at least one of carbide producing metal elements including chromium, molybdenum, tungsten, vanadium, niobium and titanium.
It is more preferable to execute the cold-warm working with respect to the component made of the steel mentioned above in a manner that the first stage carbide so crystallized are crushed in more fine size grain carbide, while cracks and voids are formed in the component, and, then, to execute the Hot Isostatic Pressing treatment on the component so that the cracks and voids formed in the component may be filled up with base material of the component. As a result, an impact strength of the component after the Hot Isostatic Pressing treatment becomes stronger than that before the hot-warm working because of change of the first stage carbide into more fine size grain carbide in the component.
As conditions of the Hot Isostatic Pressing treatment, preferably, the component is processed at a temperature of more than 900xc2x0 C., with a pressure of more than 88.2 Mpa and during more than 0.5 soaking hours in an inert gas.
Further, it is preferable to carry out a hardening treatment of the component, after the Hot Isostatic Pressing treatment for curing the cracks of first stage carbide, by adequately adjusting cooling speed of the component.